1. Field of the Invention
The present invention relates in general to magnetic separation and, in particular, to improved methods and apparatus for making continuous magnetic separations of flowable mixtures of particles of different magnetic susceptibilities.
2. Background of the Prior Art
A ferromagnetic body magnetized in a magnetic field exerts either attractive or repulsive magnetic forces on particles in its vicinity depending on the position and the susceptibility of the particles. For example, the direction in which magnetic force urges paramagnetic and ferromagnetic particles is opposite to that in which it urges diamagnetic particles. In the following description, the effects of magnetic force on particles having appreciable positive magnetic susceptibility, such as paramagnetic and ferromagnetic particles, is considered. The word "magnetic," as hereinafter used to describe particles, should be understood to mean particles having appreciable positive susceptibility unless otherwise specified. The word "nonmagnetic", as hereinafter used to describe particles, should be understood to mean particles which are diamagnetic or which have positive susceptibility too weak to be exploited for separation purposes.
Magnetic flux concentrates in a ferromagnetic body and in regions adjacent to the opposite sides where it enters and leaves the body, i.e., in the poles induced in the body. Attractive magnetic forces approximately aligned with field direction and directed toward the ferromagnetic body arise in the regions of concentration of magnetic flux, while repulsive magnetic forces arise at the other sides of the ferromagnetic body, i.e., in the spaces between the regions of attraction, and are oriented roughly perpendicular to magnetic field direction. In these regions of repulsion, the field intensity is below the average value of the field and field gradients increase for a slight distance from the ferromagnetic body and then decrease with further distance from it. The repulsive forces in these regions act substantially at right angles to the surface of the ferromagnetic body. While the attractive magnetic forces are strongest at the polar regions of the surface of the ferromagnetic body, the repulsive magnetic forces are strongest at a short distance away from the surfaces of the ferromagnetic body between the polar regions.
Most of the known magnetic separation techniques are based on the attraction of magnetic particles to a ferromagnetic body matrix. Thus, repulsive magnetic forces generated in the matrix are incidental to the separation process. Because, in many of these known separators, the magnetic particles must be washed off the ferromagnetic bodies, the material must be fed in batches. In other words, feeding is interrupted periodically.
In a prior art technique, the matrix comprises an array of elongated ferromagnetic bodies which are disposed parallel to each other and spaced apart from each other and disposed in a plane perpendicular to the magnetic field. Material fed to the matrix flows toward the ferromagnetic bodies so that nonmagnetic particles pass through the spaces between them, while magnetic particles are attracted to them and held on their surfaces. The ferromagnetic bodies must be withdrawn from the magnetic field and washed to recover the magnetic fraction.
In this method, the magnetic field is oriented with respect to parallel ferromagnetic bodies so that regions of attractive magnetic force arise at the surfaces where magnetic flux enters and leaves the bodies, whereas repulsive magnetic force directed away from the surfaces between the regions of attractive force arise in the spaces between the ferromagnetic bodies. Thus, the material approaching the ferromagnetic bodies first enters regions of attractive force, where magnetic particles are attracted to the ferromagnetic bodies while nonmagnetic particles pass through the spaces between them and are concentrated as the nonmagnetic fraction. Magnetic particles not attracted to the ferromagnetic bodies which enter the spaces between them are not separated by repulsive force because means are not provided to collect and discharge them.
This technique is embodied in a device which includes a magnetic circuit and a rotor supporting arrays of parallel elongated ferromagnetic bodies disposed in a plane perpendicular to field direction. Material is fed through this array of ferromagnetic bodies, where magnetic particles are held, while nonmagnetic particles pass through the spaces between them. However, the rotor, which is essential to the operation of the device, diminishes its reliability while increasing size, weight and power consumption.
Similarly, another technique employs grids of spaced-apart parallel elongated ferromagnetic bars inclined at an acute angle to the vertical exit direction and disposed in a plane parallel to field direction and magnetized in a horizontal magnetic field. When material is fed to the separator, magnetic particles are captured on the bars of the grids, while nonmagnetic particles pass downward through them. Continuous feeding is obtained by employing means to move a succession of matrices continuously in and out of the field.
Magnetic separation techniques based on repulsion of magnetic particles by a ferromagnetic body in a magnetic field have been reported. Generally, the material is fed to separators continuously since magnetic particles do not have to be washed off the ferromagnetic bodies. These magnetic separation methods are hereinafter sometimes referred to as continuous methods.
A prior art method of continuous separation of weakly magnetic materials by means of repulsive magnetic forces includes providing a magnetic field perpendicular or nearly perpendicular to elongated ferromagnetic bodies positioned in a plane which is parallel to field direction and adjacent to the separating chamber, feeding material into the regions of repulsion adjacent to the ferromagnetic bodies and moving the material within said regions along the length of the ferromagnetic bodies in order to deflect magnetic particles away from the ferromagnetic bodies by the action of the repulsive magnetic forces, and continuously removing the separated fractions, the magnetic particles into collection means positioned at a distance from the ferromagnetic bodies and the nonmagnetic particles into collection means positioned near the lower ends of the ferromagnetic bodies.
However, the repulsive magnetic force in a region of repulsion near the surface of a ferromagnetic body is typically about a quarter as strong as the attractive magnetic force in a region of attraction at the surface of the same body. In addition, the repulsive force becomes weaker with distance from the ferromagnetic body. Therefore, the repulsive magnetic force is too weak to separate the streams of magnetic and nonmagnetic particles sufficiently to prevent cross contamination.
Another prior art method for the continuous separation of weakly magnetic materials by means of repulsive magnetic forces includes providing a magnetic field perpendicular or nearly perpendicular to elongated ferromagnetic bodies positioned in a plane which is parallel to the field direction adjacent to and below the separating chamber, feeding material into the regions of repulsion adjacent to the ferromagnetic bodies and moving the material within said regions along the length of the ferromagnetic bodies in order to deflect magnetic particles away from them and upwards from the lower edge of the field, while the nonmagnetic particles sink towards said lower edge, and continuously removing the separated fractions, the magnetic particles passing out of the field above a mechanical divider and the nonmagnetic particles passing out of the field below the divider.
This prior art method of continuous separation, because it employs repulsive magnetic force to lift particles only a few millimeters away from the lower wall of the chamber to which nonmagnetic particles sink, is capable of effecting separation with less cross contamination than is the method previously described. Reports acknowledge, however, that some cross contamination occurs. This is attributable to the fact that the floor of the chamber prevents the material to be separated from entering the region of maximum repulsive magnetic force, where weakly magnetic particles could be deflected away from nonmagnetic particles moving under gravitational force.
Finally, another prior art method and apparatus employs a locus of maximum magnetic energy gradient, HdH/dX, transverse to the field direction at the midplane of a gap between matched pole pieces, which serves as a magnetic barrier. Material is fed onto the midplane so that it moves under gravitational or other nonmagnetic force toward the magnetic barrier, where particles having susceptibilities above a selected value are deflected along its length, while particles having susceptibilities that are lower or of opposite sign pass through the barrier. With this method, ferromagnetic particles can also be separated continuously at the barrier according to differences in their magnetic properties, since magnetic force aligned with field direction is weak compared with transverse magnetic force.
Limitations on processing capacity result because conditions for separation are only optimal at the midplane between the pole pieces. Transverse magnetic force decreases and magnetic force aligned with field direction increases as particles approach a pole piece. Thus, while feeding of material through the barrier field as a thin, well dispersed stream is consistent with good separation, the apparatus' efficiency drops with thicker, less dispersed streams.